In a GUI, an icon is a small image associated with an action, an idea, or just a thing. Icons are supposed to be easier to understand, as they present a concept without using words. So you don't even have to internationalise them. Icons have been used to denote all of the following (and probably more):

in toolbars, found on many modern applications.
Running instances of programs
in many window managers under the X window system.
Runnable programs and files
in 32-bit Windows systems
see e.g. the trashcan in Macintoshes and 32-bit Windows systems: dragging an object to the trashcan does [something].

So much for theory. Some simple icons are readily comprehensible: clicking a picture of a printer might reasonably "print" something (even here, though, should I click it, or should I drag and drop the file I want to print onto the printer, like I do with the trashcan? -- it depends where this printer is).

But what does half a ship's rudder wheel mean, if I don't remember Netscape used to be called "Navigator", and had that image? What if I fail to recognise the picture? How many pictures must I learn to recognise -- one for each action? Isn't that why we stopped using hieroglyphs and switched to letters?

Today, almost every icon has a textual accompaniment. Some icons take more screen space than the equivalent text. But they remain icons of user friendliness.